what do you mean by after a while I can not anymore having the focus on my drawing ?

Let's note that under the viewer all changes done in a diagram are lost when you close it, this means a diagram created under the viewer is emptied when it is closed (not when it is minimized). This is indicated in the warning box shown when your start the viewer.

I was making a pretty big use case diagram with about 8 to 10 actions linked as included or extended. With that kind of diagram we often need to move ansd recenter actions and actors in the drawing before saving the dagram as a .png.

But suddenly it was not possible to move one item or even to click right to save the diagram as a .png.

But the objects are still on the left part of the viewer but seems not usable.

Except a bug this is because you edit an element like an actor, the associated separated window is probably under the main window and you don't see it, please check and give the result. Additional question : is the refresh still working ? for instance minimize (iconify) the main window then open it again to see if the refresh is active. Is the boumlViewer process loops abnormally (get 100% CPU time) ?